Due to the development of mobile technology and the rapid increase in demand for mobile applications, demand for secondary batteries has also rapidly increased, and, in particular, lithium secondary batteries are now widely used as a power source for various electronic applications including mobile devices, due to its properties such as high energy density, high operating voltage, and excellent endurance and lifetime.
However, since the lithium secondary battery contains various flammable materials, there are disadvantages such as heat generation and explosion caused by over-charge, over-current and external physical impact, leading to instability of the battery. In order to prevent this problem, the lithium secondary battery has a protective circuit module (PCM), which is embedded therein and connected to the battery cell in order to effectively suppress abnormality of the battery such as over-charge.
The PCM comprises a field effect transistor acting as a switching device for controlling electric current, and various passive devices such as a voltage detector, a resistor, and a capacitor. The PCM suppresses over-charge, over-discharge and over-current of the battery, thereby preventing explosion or overheating of the battery, leakage of liquid, and deterioration in charging or discharging properties. In addition, the PCM suppresses reduction in electric properties, and physical and mechanical abnormality of the battery. As a result, the PCM acts to enhance the lifetime of the battery while eliminating dangers thereof.
In FIG. 1, an assembling method for electrically connecting the PCM to the battery cell in a conventional lithium secondary battery is illustrated.
Referring to FIG. 1, the battery cell 1 comprises a protruded anode (negative) or cathode (positive) terminal 1a (herein, an anode terminal is illustrated) formed on an upper end thereof, and an outer case constituting a cathode or anode terminal as an opposite electrode. The battery cell 1 further comprises two electrode leads 2 and 3 for electrical connection between the battery cell 1 and a PCM 4. The cathode lead 2 is welded to the upper end of the battery cell 1 and a cathode tap (not shown) provided at a lower end of the PCM 4, while the anode lead 3 is connected to the anode terminal 1a of the battery cell 1 and an anode tap (not shown) provided at the lower end of the PCM 4. In order to prevent electrical short from occurring by contact between other portions of the anode lead 3 connected to the anode tap and the PCM 4, an insulating paper 5 is interposed therebetween.
In this structure, several welding steps are required for ensuring electrical connection between the battery cell and the PCM. That is, it is necessary to weld the respective electrode leads 2 and 3 to the electrode terminals of the battery cell 1, and then to weld the electrode leads 2 and 3 to the electrode taps of the PCM 4. Furthermore, due to light weight and compactness of the secondary battery, components of the battery are further reduced in size, and thus welding of such small components requires skilled technique or precision equipment, thereby complicating the assembly process. As a result, lots of time is required for the assembly process, and the frequency of defective assembly is increased. Moreover, in order to weld the electrode leads 2 and 3 welded to the battery cell 1 to the electrode taps of the lower end of the PCM 4 again, a groove must be formed on a side surface of a battery cap (not shown) so as to provide an approach to the associated position from the side surface. However the groove is also used as an inlet of foreign substances (for example, water), causing malfunction or damage of the battery.